I'm assuming we are still talking about the .075 " test pieces, if so prepare a simple butt joint with two small pieces approx. 4" x 4", weld the two together,(you only need around 1" of weld here) then fold the test onto itself using the point where they were butted (excuse the grammer), as an axis. The weld you deposited should be IN the bottom of what now looks somewhat like a V , continue bending until the piece is folded onto itself or the weld or base metal breaks. Oh yeah if the piece is folded all the way onto itself with no signs of the weld breaking, it's a beautiful thing. If the base metal breaks, also a beautiful thing. If the weld metal breaks OMG, no just kidding, not as beautiful but not necesarily failing, your out of a job, depends on whats inside a failed weld, all this per AWS D1.3 1998

also, for a fillet (T-joint) weld you can weld one side and then fold the two pieces together TOWARD the weld...never fold away from the weld because you want to break the bead to see if there is complete fusion. bend the two halves back and forth until they break apart. it will be hard to see if you have complete fusion on your thickness of metal, but it can be done. if you can still see the original corner of the metal on the welded side, you have lack of fusion. if the edge is jagged and looks torn, you have complete fusion. also, look for air pockets and porosity...a weld will always break along the weakest points.

You can also take a joint just like the one you are doing in production, and after welding it, saw it in half with your band saw, chop saw, cut off wheel, or whatever is handy. Then take a nice fine grit flapper wheel on your 4 1/2" grinder, and get it as smooth as you can. Sometimes after this, you can see the weld fairly good, and can evaluate what kind of fusion you have. If you can't see the weld very good, the usual method is to etch with nitric acid, but you could experiment with what you have at home to see if you can get the weld to show up. If you try something and it doesn't work, re-polish with the flapper wheel, and try again.